Otsego Lake possesses the steep shores and bathtub-shaped basin typical of glacially overdeepened river valleys. Figure s 7 and 8 are reproductions of bathymetric maps (Harman 1974a). Calculations from the original are indicated in Table 16.
The lake basin was gouged from the shales and siltstones of the Panther Mountain Formation (Middle Devonian) and lower Hamilton members of the Hamilton group. A prominent dip slope on the Onondaga limestone (Lower Devonian) is exposed on the floors of the valleys at the north end of the lake and forms much of the northern end of the basin as it dips gently south by southw est under the lake surface at a rate of about 17 m/km (Figure 9, Sales et al., 1978).
The longitudinal geometry of the lake basin is dominated by a shallow terrace at about 5 m depth, a bench face to a depth of 10 to 15 m, a second terrace between 11 and 20 m, a steeper slope between 20 and 45 m, with a comparatively flat bottom at the greatest depths (45-50m) (Figure 8). One hypothesis is that the terrace at the north end of the lake is the extension of the Onondoga dip slope, the bench face below is the glacially plucked lee slope of these limestones, while the slope at 11-20 m is the glacially smoothed and subdued Helderberg limestone under a veneer of bottom sediments (Sales et al., 1975).
These phenomena took place during several sequences of advancement and melting away of the tongue of ice that overdeepened the valley creating the contemporary Otsego Lake basin (Fleisher and Mullins, 1990). Figures 10a and b correlate a seismic interpretation of the bedroc k and soils under the lake with on-land data from the Cassville-Cooperstown moraine under the Village at the south end of the lake (Fleisher and Mullins, 1990).
Lake sediment geology has been studied by Sales et al. (1975; 1978), Clikeman (1979), and Yuretich (1980; 1982). Three sedimentary groups are dominant. Waters one to two meters in depth at the northern end of the lake are underlain by laminated calcareous marls with carbonate contents of 40 to 60 percent. Gastropod fossil s are abundant and there is local development of algal stromatolite nodules (Yuretich, l980). Waters 10-50 meters in depth in the central and southern portions of the basin are covered by a dark grey, organic muddy silt which is largely unlaminated. Carbonate content drops to 20-40%, and organic carbon is consistently around 3 percent. There are few crystalline clay minerals and a large percentage of amorphous iron oxhydroxide is present. In the western shallows, a cohesive, steel-grey clay lies under neath a thin veneer (5-30 cm) of muddy silt or marl. These very fine grained sediments are 61% clay and low in both organic carbon and carbonate. Illite and kaolinite are abundant (Yuretich, 1980).
The distribution of sediments is apparently controlled by solubility relationships in the water above. Deep waters are undersaturated with respect to calcite while the shallow waters above are often supersaturated. The gray clays along the western shore are similar to Pleistocene clays north and west of the lake. They may be a remnant of Pre-Otsego glacial Lake Cooperstown (Yuretich, 1980).
Littoral surficial sediments are composed of limey gravelly clay, with the inclusion of many gastropod shells. The greater than 1mm sieve fraction is composed of 39% fine grained sandstone and siltstone; 16% weakly consolidated brown silty clay clasts; 14% twigs and other plant fragments; 8% slag (derived from steamboats); 2% chert and 1% quartz and metamorphic rock fragments. Core analyses reveal that slag first appears in the cores at a level corresponding with an increase in sandstone, siltstone, chert, and quartz. This indicates that heightened human activities in the latter half of the 1800s is responsible for these changes in the sediment composition (Sales et al., 1978). Additional data on sediment chemisrty is presented in the "Macrophyte" section (see Table 34).
Surficial substrates are dominated by clays and silts throughout the entire basin below the level of wave disturbance (Figure 11). Typically, they are oxygenated and grey in appearance for the upper 1 to 2cm. Below this depth a reducing environment is encountered, typified by black soils smelling faintly of sulfide compounds. Fine sand, gravel, cobbles, channery and exposed bedrock appear along the shore at various places, as illustrated in Figure 12. The sands are derived from fluvial deposits in the valleys of Shadow Brook and Hayden Creek, the cobbles and boulders from glacial deposits, while most gravels are derived from shales (channery) exposed along the shorelines. In waters from 1 to 10 m in depth, exposed bedrock often occurs, forming practically vertical walls dropping into the main basin.